Formation of high modulus,high strength graphite yarns



United States Patent 3,528,774 FORMATION OF HIGH MODULUS, HIGH STRENGTH GRAPHITE YARNS Herbert M. Ezekiel and Raymond G. Spain, Dayton, Ohio, assignors to the United States of America as represented by the Secretary of the Air Force N0 Drawing. Filed Mar. 14, 1967, Ser. No. 623,520 Int. Cl. C01b 31/07 U.S. Cl. 23-209.1 2 Claims ABSTRACT OF THE DISCLOSURE A method for forming high tensile strength, high elastic modulus graphite strands, particularly from polynuclear aromatic polymeric precursory materials comprising one or more of the steps of pretreating the precursor as by preoxidation, carbonizing as by the subjection thereof to temperature of up to 1500 degrees centigrade, graphitizing by exposure to still higher temperatures of from 1800 degrees to 3200 degrees centigrade, and/or orienting the crystalline graphite structure by some means during graphitization.

The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to us of any royalty thereon.

BACKGROUND OF THE INVENTION Field of the invention The present invention pertains to the formation of high tensile strength, high elastic modulus graphite yarns and particularly of continuous yarns of multiple filaments, either as strands or in fabrics.

Description of the prior art Steady increase in the performance capabilities of aerospace vehicles and devices and in their resultant exposure to increasingly severe temperature and oxidative environments, often combined with subjection to dynamic shear or other mechanical eroding stresses, have created considerable demand for more efficient structural materials. More specifically, this demand has been for improved reinforcing materials to be embodied in or otherwise incorporated with a variety of known ablative, plastic or metallic matrix compositions which have been regularly employed in the construction of aerospace components such as the leading edges of high speed aircraft, the nose cones or heat shields for atmospheric escape and re-entry vehicles, rocket engine components, particularly the combustion, nozzle and exhaust components thereof, and the like. Because weight is almost always a critical factor in the design of such structures and devices, the development of composite materials with high strength and high modulus of elasticity-to-weight ratios has been especially sought.

Because graphite is Well known for the substantial retention of its favorable mechanical properties to about 2,000 degrees centigrade, particular attention has been given to the manufacture of graphite strands or fibers in a variety of forms suitable for use as reinforcements in composite materials for aerospace structures. Various methods for the production of such graphite reinforcing materials have included whisker growth and the deposition of pyrolytic graphite on metal filament substrates. Problems inherent in these prior art methods and techniques have, however, been substantial deterrents to the widespread use of graphite reinforcing components and have seriously limited the full exploitation of the unique structural capabilities which graphitic materials are known 3,528,774 Patented Sept. 15, 1970 to have in connection with aerospace applications. Included among such problems and disadvantages are the cumbersome and expensive characteristics of the apparatus required for either continuous or batch pyrolytic pro duction. The equipment now in use is not only exceedingly expensive in the first instance but is expensive to maintain. Moreover, it requires excessive expenditures of man hours for the production of relatively limited amounts of the graphitic material. The precision required in supplying the controlled atmospheres which are critical to the formation of graphite fibers or strands also adds to the cost and the time required for their production. Contributing to this is the inherently slow crystalline growth rate typical of whisker-forming operations. At the same time, those prior art techniques involving the use of a filamentous substrate upon which the graphitic material is deposited in strand formation have, because of the necessity of the substrate, been capable of producing only relatively large diameter fibers, as a result of which the desired fiexilibity has been unattainable.

In view of the foregoing disadvantages, it has been recognized that it would be desirable to employ a preformed polymeric precursory fiber or strand of the desired dimensions in the formation of graphite by the conversion of such precursor into a carbonaceous residue which might then be converted into graphite. This has led to considerable development along the lines of the carbonization and ultimate graphitization of a variety of natural and man-made cellulosic fibrous materials such as cotton, kapok and other flosses, pine and other woody fibers, straw, flax, hemp, sisal and a variety of regenerated cellulosic fibers such as those of viscose rayon, cuprammonium rayon and saponified acetate rayon. While the subjection of those materials, either in the form of individual fibers, as strands or as woven fabric, to high temperature decomposition has resulted in their conversion at least partially into carbon or graphite yarns and fabrics, the handling properties of the finished products have been undesirable in many particulars, especially in that they have been subject to unexpected and premature breakage during the manufacturing stage or their incorporation in composites. Moreover, such treatment of the cellulosic materials has produced relati-vely low carbon yields as a result of which the expense of manufacture has been relatively high, the supply has been relatively limited and the weight factor has been troublesome, particularly in aerospace applications. Further complicating the prior art procedures has been the fact that the best graphite fibers and yarns produced to date have required a carefully controlled stressing of the precursory material during carbonization and/or graphitization.

While the nature and causes of the foregoing disadvantages are not completely understood, it has been found upon experimentation that both the natural and manmade fibers are characterized by substantial inhomogeneities in their original condition or state; and it is believed that these have interfered with the uniformity, predictability and reliability of the finished reinforcements and of the composite materials of which they form a part. It is recognized further that the carbonization or graphitization of the cellulosic fibers requires a breaking up of the cellulosic molecule and a reconstitutional rebuilding thereof which not only adds to the time and cost involved in the manufacture but also introduces a variety of critical factors, the control of which for a satisfactory product is often unachievable.

Of greater significance than overcoming all of the above enumerated disadvantages and deficiencies of the prior art however, is the fact that the material made possible according to the teachings of the present invention is a truly high strength, high modulus graphite strand in continuous form. The fact that continuous strands having elastic moduli as high as twenty million pounds per square inch, tensile strengths as high as 58,000 pounds per square inch and relatively high degrees of purity of graphite content are, by this invention, now made available from a relatively wide range of precursory materials is a most significant contribution of the invention described and claimed herein.

SUMMARY OF THE INVENTION It is accordingly an object of this invention to provide ultra high tensile strength, ultra high elastic modulus graphite yarns, and particularly continuous yarns or fabric woven thereof.

It is still another object of the invention to provide such yarns and fabrics which may be produced at a relatively low cost by relatively uncomplicated techniques.

Still another object of the invention is the provision of such yarns and fabrics which are characterized by a higher yield of carbon or graphite.

Still another object of the invention is the provision of such yarns and fabrics which can be handled without breakage and will be amenable to incorporation as reinforcements in a variety of matrices to form composites which, because of their strength-to-weight ratios, are suitable for use in aerospace applications.

To achieve these and other objects and advantages which will appear from a reading of the following disclosure, the present invention teaches the utilization of a variety of polymeric precursory yarns and fabrics not heretofore considered in connection with the formation of graphite. The polymers to be so employed have been found to be the polynuclear aromatics which upon thermogravimetric analysis exhibit particularly high char residues, the class of which consists of poly[2,2'-(m-phenylene)-5,5'-(dibenzimidazole)], poly[1,3/ 1,4 phenylene-2,5-(1,3,4-oxadiazole)], poly[(1,3/1,4 phenylene)-2,5(1,3,4-thiadiazole)], poly(bis benzimidazobenzophenanthrolene), the aromatic polyamides and the aromatic polyimides. These polymeric precursors are subject to high temperature processes which break down the polymeric molecular structure leaving a carbonaceous residue which in turn is then converted to a pure graphite structure.

In the formation of the high elastic modulus, high strength graphite fibers, filaments or fabrics from the above-enumerated polymers, the conversion may be a continuous or a batch process consisting of some or all of the steps of: (l) pretreating the precursory polymer such as by the preoxidation thereof in air, ozone or other chemical agents in the form of gases, solutions, liquids or solids; (2) thereupon carbonizing the polymer as by the subjection thereof to temperatures of as high as 1500 degrees centigrade; (3) graphitizing of the carbonaceous residue by subsequent exposure of the carbonized material to temperatures of from 1800 degrees to 3200 degrees centigrade; and/or (4) orienting the crystalline graphite structure by some means such as the application of tensile stress along the fiber axis during graphitization. Typical means for carbonizing and graphitizing the polymer may involve the continuous passage of the precursory material through a furnace capable of providing the requisite temperatures in an inert atmosphere such as of argon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In one specific embodiment of the invention a 200 denier, 50 filament poly[2,2-(m-phenylene)-5,5-(dibenz imidazole)] yarn with a slight twist was formed into a six-ply yarn with approximately two turns per inch. This yarn was oxidized by passage in air through a tubular furnace at from 445 to 475 degrees centigrade for approximately seven minutes while maintained under tension by the suspension from the yarn of a 195 gram weight. Some of the yarn thus oxidized was then carbonized while under slight tension in a furnace with two argon-filled heating zones, the first being maintained at a temperature of from 590 to 625 degrees centigrade, and the second being maintained at a temperature of 1,010 degrees centigrade. Exposure of the yarn to the temperatures within each of the zones for approximately three minutes resulted in its carbonization. At this point the average single fiber tensile strength was found to be on the order of 25,000 pounds per square inch and the initial modulus of elasticity was on the order of 9,800,000 pounds per square inch. Another sample of the oxidized polybenzimidazole yarn was carbonized in the same furnace with the temperature zones maintained at 615 degrees and 1,360 degrees centigrade respectively. The higher second stage temperature resulted in a carbon yarn with an average single fiber tensile strength of 97,000 pounds per square inch and an initial modulus of elasticity of 11,900,000 pounds per square inch.

The sample of the polybenzimidazole yarn which had been carbonized at the maximum temperature of 1,010 degrees centigrade (in the second stage of the carbonization furnace) was passed through a heated zone at temperatures of from 2,725 to 2,750 degrees centigrade while maintained under very slight tension. Exposure of the yarn to this temperature for approximately one-half minute resulted in conversion of the yarn to substantially pure graphite yielding an average single fiber tensile strength of 58,000 pounds per square inch and an initial modulus of elasticity of 20,000,000 pounds per square inch.

As an example of the formation of high modulus, high tensile strength strands according to the present invention from polyoxadiazole, a 60-filament yarn of poly[1,3/1,4- phenylene-2,5-(l,3,4-oxadiazole)] was passed through a 14 inch tubular furnace containing air at 455 degrees centigrade with a 15 gram weight suspended from the yarn by a 0.7 gram rider pulley. The control of the yarn movement was such that it passed into the furnace at about 1.9 inches per minute and emerged at the rate of 3.6 inches per minute resulting in an 89 percent elongation of the yarn.

A portion of the yarn thus oxidized was then carbonized in an inert atmosphere such as lampgrade nitrogen by passing at the rate of nine inches per minute through a three-stage furnace under the nominal tension resulting from a 0.75 gram weight suspended from the yarn. Each of the furnace stages or zones was approximately six inches in length. The first zone was maintained at a temperature of from 440 to 445 degrees centigrade; the second stage was maintained at from 560 to 580 degrees centigrade; and the third and final stage through which the yarn passed was maintained at from 715 to 720 degrees centigrade. Thereafter, the yarn thus carbonized was graphitized by heating in argon to approximately 2,550 degrees centigrade by passage through a vertical graphite susceptor. The exposure of the yarn to the graphitization temperature in the susceptor was less than a minute and only a nominal weight was suspended from it. The product was a stiff, grayish-black yarn from which the separation of individual filaments was extremely difiicult. Tensile tests however were run upon one-half inch specimens which varied from possible individual fiber filaments to tightly compacted bundles, typical samples of which were found to have the following properties. A 0.55 mil diameter sample had an initial elastic modulus of 10,300,000 pounds per square inch and a tensile strength of 35,000 pounds per square inch. A 0.84 mil diameter sample had an initial elastic modulus of 5,000,000 pounds per square inch and a tensile strength of 19,000 pounds per square inch. A 0.67 mil diameter sample had an elastic modulus of 10,300,000 pounds per square inch and a tensile strength of 54,000 pounds per quare inch; and a 1.74 mil diameter sample had an elastic modulus of 2,300,000 pounds per square inch and a tensile strength of 11,000 pounds per square inch.

The degree of improvement in the physical properties and graphitic content of the continuous filaments or strands according to the present invention is illustrated by comparison of the above properties with the best of those that were heretofore obtainable by graphitization according to prior art techniques. For example, continuous strands formed by the graphitization of cellulosic precursory yarns such as those composed of rayon were characterized by elastic moduli of on the order of from four to five million pounds per square inch and tensile strengths of from 50,000 to 100,000 pounds per square inch.

While the within invention has been described in considerable detail in connection with certain specific embodiments and examples thereof, it is to be understood that the foregoing particularization has been for the purpose of illustration only and does not limit the scope of the invention as it is defined in the subjoined claims.

We claim:

1. A method for the formation of an ultra high tensile strength, ultra high elastic modulus graphite strand comprising the steps of oxidizing poly[2,2-(m-phenylene)- 5,5'-(dibenzimidazole)] yarn by passage thereof through air maintained at a temperature of from 445 degrees to 475 degrees centigrade, for approximately seven minutes while under tension, carbonizing the yarn thus oxidized by successively passing it through two zones of heated argon, the first at a temperature of from 590 to 625 degrees centigrade and the second at a temperature of 1,010 degrees centigrade at a rate which exposes the yarn to each heated zone for approximately three minutes, and then passing the yarn thus carbonized through a zone heated to a temperature of from 2,725 to 2,750 degrees centigrade under slight tension for approximately thirty seconds.

2. A method according to claim 1 in which said oxidizing step is carried out with said yarn under a tension of grams.

References Cited UNITED STATES PATENTS 3,094,511 6/1963 Hill et al 260-78 3,174,947 3/1965 Marvel et al. 26047 3,179,634 4/1965 Edwards 26078 3,357,956 12/1967 Frazer 260-79 3,410,834 11/1968 Pruckmayr 260-78.4

FOREIGN PATENTS 3,285,696 11/1966 Tsunoda 23-209.1 3,412,062 11/1968 Johnson et a1. 23-209.1 X 3,427,120 2/1969 Shindo et a1. 23-2091 X 3,441,378 4/1969 Didchenko 23209.1 3,449,077 6/1969 Stuetz 23-209.1

EDWARD J. MEROS, Primary Examiner US. Cl. X.R. 23209.2 

